I am playing a mage LVL 12, that uses destruction and restoration in vanilla Skyrim.
Tell me, are DG vampire lords strong as mages and is MQ requaired?

I would like to know this also, because I plan on having a werewolf mage that uses Sneak, Destruction, Illusion, and Archery for his crossbow at far distances. Oh, and a little bit of Alteration for Mage Armor spells.

1st question yes or better than mages.
2nd question yes if you want a stronger more super awsome vampire lord.
And im mean the MQ for dawnguard not skyrim.

No I plan on being on the Dawnguard side. So what about that with my mage character?

Yes VL are strong as mages and It does require you to go through dawnguard's main quest.
Well you get some new restoration spells.

Ehh your going to get some new spells but IMO not as great as op vampire lord.

just a side note for all:

Of course how can I forget. You also get some new conjuration spells.

You get two new restoration spells if you join Dawnguard, and correct me if I'm wrong, and one if you join the Vampires. Stendarr's aura creates a sphere of light around you, harming any yndead that come into melee distance. Vampires bane has an AOE and once cast, also harms undead around you. Heal undead does what it says on the tin.

Although I am a PS3 user and don't know from experience, I believe a Magic based character would be better as a VL and a combat based with archery and either one or two handed would be best for Dawnguard... but ya know what... that is the beauty that is TES - you can do whatever you want. So if you wanna fight those vampire's fire with fire Then do it!